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In our experience assisting users, the most often seen cause is that one or more items do not have observations in specified response categories. For example, flexMIRT® flexMIRT™ is told that an item has 4 categories and should fit the Graded(4) model but, in the data, there are not observations that have a value of 2. It is impossible for flexMIRT™ to estimate an intercept between two categories if one of the categories is not seen in the data. Collapsing/recoding the data so observed response values are sequential and specifying an item model with an appropriate number of categories is one way to resolve the problem.
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Currently there is not an option to save the original parameterization values into the -PRM file, though they are printed in the output. Using the alpha and gamma values saved into the -PRM file and following the formulae in Thissen, Cai, and Bock (2010), specifically the T matrix described in Eq. 3.35 and 3.36 and parameter conversions found in Eqs 3.37 – 3.41, users may obtain the estimates in the original parameterization. Please note that there is an error in Eq. 3.41 in the chapter, which should read:
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I fit a multidimensional model and
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flexMIRT™ is only printing the intercept values (c parameters). I would like thresholds (bs) instead. Can I make flexMIRT™ print those?
As a logistic multidimensional model, threshold parameters are not straight forwardly derived and tend to have far less logical and theoretical basis; for these reasons, flexMIRT™ will not print them. However, you can convert the logistic intercepts into approximate Normal metric threshold parameters using the following method:
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